1. Field of the Invention
The present invention relates to an automatic focus state detecting apparatus for use in a camera or the like.
2. Related Background Art
There is already known an automatic focus state detecting apparatus having a relatively narrow first focus state detecting area and a wider second focus state detecting area, both in the photographing field and either of which is selectable by the photographer, thus detecting the amount of defocus of the image plane of a photographing optical system with respect to a predetermined plane in the selected focus state detecting area, and moving the photographing optical system to the focused position based on thus detected amount of defocus.
However, such automatic focus state detecting apparatus the following drawbacks in either detecting area:
1) When the relatively narrow first focus state detecting area is selected by the photographer:
Because the area is relatively narrow, a stationary or slow-moving object can always be caught in the area without presence of an undesired object, but it is difficult to hold a fast-moving object in the detecting area. If the object runs out of the detecting area, the apparatus becomes unable to detect the focus state, thus resulting in unnecessary scanning motions of the lens, or focusing to other objects in the background.
2) When the relatively wide second focus state detecting area is selected by the photographer:
It is easier to retain even a fast-moving object always in the detecting area, but, in case of a stationary or slow-moving object, the possibility of presence of other objects becomes higher as the detecting area is wider. If the area contains another object which is not desired by the photographer, the lens may be focused to such another object.
Also the Japanese Patent Application Sho 63-17418 discloses an automatic focus state detecting device capable of forming, in the photographing field, a relatively narrow first focus state detecting area and a wider second focus state detecting area. In response to a half push of the shutter release button, the apparatus selects the narrower first area, and, if the focus state is undetectable, the wider second detecting area is selected. The apparatus detects the amount of defocus of the image plane of the photographing optical system with respect to a predetermined plane in the selected detecting area, and moves the optical system to the focused position based on the detected amount of defocus.
However, such automatic focus state detecting apparatus still has the following drawback. If the focus state detection is not possible because of low contrast of the image of the object formed on the focus state detecting device such as a CCD, it is still difficult to detect the focus state of the object in the wider second detecting area as the desired object is often affected by other objects of higher contrast.
Furthermore there is also known an automatic focus state detecting apparatus capable of so-called following drive for moving a photographing lens without delay to a moving object. Such apparatus repeatedly detects the amount of defocus of the photographing lens, and discriminates whether the object is moving, from the present and past amounts of defocus. If the object is moving, it determines a correcting value, based on the present and past amounts of defocus, for correcting the position of the photographing lens toward the moving object, and moves the photographing lens based on the amount of defocus and the determined correcting value. There is further known an automatic focus state detecting apparatus of such following drive type, combined with narrow and wide detecting areas mentioned above.
However, such conventional apparatus has the following drawback, because the following drive and the switching of detecting area are conducted independently. When the following drive operation is started in the narrower first focus state detecting area, the photographer has to change the position of the camera according to the movement of the object, in order to retain the moving object in the first detecting area, but this operation is difficult because the first detecting area is narrow. If the object escapes even momentarily from the first detecting area, the following drive is interrupted because the focus state detection is made on other objects in the background or becomes impossible, and smooth following drive cannot be achieved for this reason. On the other hand, when the wider second focus state detecting area is selected, it is easier to hold the object in that area because it is wider. However, if the main object starts to move from a stationary state, it is difficult to discriminate whether the object is moving because the detecting area contains other stationary objects, so that the following drive cannot be started immediately, without delay with respect to the movement of the object.